Matter's Erika Hayasaki visited Mexico's National Institute of Neurology and Neurosurgery to learn about Cotard's Syndrome.

This terrifying disorder turns people into zombies, into living, breathing ghosts; they believe they died, or never existed. And somewhere in their brains may be the key to human consciousness.

[...] Most of the patients here have been diagnosed with garden-variety neurological disorders: schizophrenia, dementia, psychosis, severe depression, or bipolarism. But the ones I am searching for are different. They suffer from an affliction even more puzzling: They believe that they are dead.

It’s a rare disorder called Cotard’s syndrome, which few understand. For patients who have it, their hearts beat and lungs pump, yet they deny their existence or functionality of their bodies, organs or brains. They think their self is detached. (Source)